中子引发裂变链概率的演化过程模拟  

Evolvement simulation of the probability of neutron-initiating persistent fission chain

作　　者：王喆[1] 洪振英[1] 

机构地区：[1]北京应用物理与计算数学研究所,北京100094

出　　处：《核技术》2014年第5期62-68,共7页Nuclear Techniques

基　　金：中国工程物理研究院科学技术发展基金项目(No.2013B0103017)资助

摘　　要：在相对速度空间建立中子引发裂变链概率所满足的与时间相关的微分-积分方程,基于多群SN方法开发动态数值程序(Dynamic Segment Number Probability,DSNP),分析了动态计算的收敛性,并对动态系统的裂变链概率演化过程进行数值模拟。模拟计算表明,DSNP程序与Partisn程序的计算结果均一致;临界状态附近存在大量的有限裂变链,使得引发概率的动态演化结果高于稳态计算结果,在Baker动态流场模型上,第一临界点后1μs的范围内计算结果最大差异约为300%。随着裂变系统反应性增加,有限裂变链的贡献逐渐减弱,持续裂变链占优,引发概率的动态演化曲线与稳态结果逐渐重合,差别小于5%,表明系统中子引发自持裂变的能力趋于稳定,此时动态引发概率的时间积分结果比稳态结果高5%-35%。高浓铀模型上的数值模拟验证了DSNP程序的准确性,该程序可定量计算动态系统的引发概率,相对于稳态方法,DSNP程序能够更为准确地描述裂变系统点火概率的演化过程。Abstract Background： Probability of neutron-initiating persistent fission chain, which has to be calculated in analysis of critical safety, start-up of reactor, burst waiting time on pulse reactor, bursting time on pulse reactor, etc., is an inherent parameter in a multiplying assembly. Purpose： We aim to derive time-dependent integro-differential equation for such probability in relative velocity space according to the probability conservation, and develop the deterministic code Dynamic Segment Number Probability （DSNP） based on the multi-group SN method. Methods： The reliable convergence of dynamic calculation was analyzed and numerical simulation of the evolvement process of dynamic probability for varying concentration was performed under different initial conditions. Results： On Highly Enriched Uranium （HEU） Bare Spheres, when the time is long enough, the results of dynamic calculation approach to those of static calculation. The most difference of such results between DSNP and Partisn code is less than 2%. On Baker model, over the range of about 1 kts after the first criticality, the most difference between the dynamic and static calculation is about 300%. As for a super critical system, the finite fission chains decrease and the persistent fission chains increase as the reactivity aggrandizes, the dynamic evolvement curve of initiation probability is close to the static curve within the difference of 5% when the Kee is more than 1.2. The cumulative probability curve also indicates that the difference of integral results between the dynamic calculation and the static calculation decreases from 35% to 5% as the Keff increases. This demonstrated that the ability of initiating a self-sustaining fission chain reaction approaches stabilization, while the former difference （35%） showed the important difference of the dynamic results near the first criticality with the static ones. The DSNP code agrees well with Partisn code. Conclusions： There are large numbers of finite fission chains near

关 键 词：持续裂变链概率 动态裂变系统 多群SN方法 

分 类 号：O571.437[理学—粒子物理与原子核物理] TL329[理学—物理]